Illegality detection electronic control unit, vehicle onboard network system, and communication method

Provided is an illegality detection electronic control unit 100 connected to a bus that is used for communication by a plurality of electronic control units communicating in accordance with a Controller Area Network (CAN) protocol, wherein the illegality detection electronic control unit is provided with: a reception unit 110 for receiving a frame the transmission of which to the bus has been started; and a transmission unit 120 which, when the frame received by the reception unit falls under a prescribed condition that indicates illegality, transmits an error frame to the bus before the very end of the frame is transmitted, and transmits a normal frame that conforms to the CAN protocol to the bus after transmitting the error frame.